Iron phosphate conversion coatings are produced on a ferrous metal surface by contacting the metal surface with an acidic phosphate-containing solution which reacts with the ferrous metal surface. Iron present in the metal substrate undergoing treatment chemically reacts with the phosphating solution and redeposits on the surface to form a low weight, tightly adherent amorphous iron phosphate coating which provides highly effective corrosion resistance and serves as an excellent base for subsequent painting.
In order to form a coating of acceptable weight and quality within a reasonable period of time, it has been generally necessary for the phosphating process to be carried out at an elevated temperature of at least 120.degree. F., and usually higher. Processing temperatures on the order of 140.degree. F. to 160.degree. F. are typical for conventional iron phosphating processes.
The economic and practical desirability of producing phosphate coatings at lower temperatures has been well recognized, and the iron phosphating field has been active with attempts to produce suitable low temperature phosphating compositions and processes. In the patent literature for example, broad references are made to low temperature or ambient temperature phosphating compositions and processes in a number of patents, such as the following: U.S. Pat. Nos. 3,152,018; 3,726,720; 4,017,335; and 4,149,909.
However, to applicants' knowledge, the iron phosphating compositions and processes which are presently available have been unsuccessful in producing iron phosphate coatings of acceptable quality and/or weight under truly ambient temperature processing conditions. Some products have been available commercially that would produce iron phosphate coatings in an ambient temperature phosphating bath, but only if the part to be processed has been suitably precleaned in a heated alkaline cleaning stage prior to the phosphating bath. Thus, this type of phosphating process still requires heating and thus cannot really be characterized as an ambient temperature process. As noted in U.S. Pat. No. 4,149,909, the necessity of obtaining effective cleaning of the metal surface has required the use of elevated temperatures, and has thus been an obstacle in the development of a truly ambient temperature phosphating system.
The rapidly escalating cost and diminishing supply of heating fuel makes the need for an effective ambient temperature phosphating system all the more important, since the energy cost for maintaining the processing baths or sprays at the proper elevated operating temperature is a very substantial factor in the overall cost of the phosphating operation.
With the foregoing in mind, it is a primary object of this invention to provide a process and composition for producing iron phosphate coatings at normal ambient temperatures, e.g. from 50.degree. F. to 100.degree. F., so as to thereby eliminate any need for providing heated cleaning and/or phosphating solutions for producing these coatings and to thus significantly conserve energy and reduce manufacturing costs.